This invention relates to the operation of gas turbine engines, and in particular to determining the deterioration of components during operation. In a specific embodiment, the invention is concerned with determining the actions to be taken when a foreign body impact has been detected, for example on a fan blade.
Birdstrikes on aircraft and their engines are disruptive to service. The fan blades of a gas turbine engine are designed to withstand small birdstrikes, and are capable of flying on. Larger birds may cause damage to the fan blades, and in these circumstances the regulatory authorities require the aircraft to land for an engine inspection. The most common time for such birdstrikes is during take-off or approach to landing. Damage can also be caused by debris sucked up by engines as they travel along the runway and taxi ways. Such incidents, whether caused by birds or by debris, are commonly referred to as foreign object damage or FOD events.
Such events are disruptive to travellers, unnecessarily so if the aircraft is able to fly on with no action required following post-FOD inspection.
If a damaged blade requires replacing, the fan set will need rebalancing because the rest of the blades in the set are part worn. To assist that rebalancing, the blade diametrically opposite the damaged one is also replaced. For a large bird strike (where more than one blade may be damaged), up to six blades may have to be replaced. The replacement, rebalancing, and reassembly into the engine can take several hours.
A fan blade at take-off power may have a tip speed as high as 1400 ft/s (˜430 m/s), which can result in bird impact velocities of 850 kt (1570 km/hr) imposing a significant impact load on the fan blade.
The cost of delaying a large aircraft full of passengers until the following day could be as much as $400,000, including the adjustments needed to aircraft schedules and the need to move aircraft around to meet schedules [USDA data]. This does not include the cost and disruption to passengers, nor consequential losses. A report by USDA suggests that the total cost of birdstrikes in the USA has been around $200 million over the last 10 years.
The cost to the environment of a birdstrike is also significant. Current procedure, following a confirmed birdstrike during take-off, is for the aircraft to go around and land. Prior to landing, the aircraft must either fly in a holding pattern to burn off fuel or must dump fuel before landing. For instance, in a recent birdstrike on a 747 out of Los Angeles, the aircraft went out over the sea and dumped 83 tonnes of fuel before returning to land. Although the engines were found to be undamaged and the aircraft continued its journey, there was a significant environmental impact through the dumped fuel and the additional fuel that had been burnt.
In summary, then, there are many birdstrikes that cause no or only very minor damage, but for safety reasons the aircraft is landed, so that the engine may be fully inspected before the journey is completed. This causes disruption to flight schedules and to passengers, and financial and environmental cost.
Various methods and techniques have been used in the past to try and mitigate the problems caused by FOD events, but all have disadvantages.
In order to build up experience from birdstrikes, where damage has occurred, the damaged blades are swabbed and measured. The swabs can provide bird species identification through DNA analysis.
Single camera systems, as described in US2011/0041474, can indicate that damage is present but cannot show the extent of the damage.
Engine vibration systems can measure the effect of out of balance, but currently cannot determine the change in a single blade moment weight. They may be able to detect impacts and, by analysis, determine impact energy and duration.
Use of a single camera in a static frame can be used to determine damage, as this is effectively only a projection in a single plane, but not all forms of damage can be detected. Using a 3D optical metrology system, such as those supplied by GOM, allows a better analysis of damage to be made, but such devices are generally designed to be used on stationary, rather than moving, components.
U.S. Pat. No. 7,399,158 describes a technique for detecting FOD entry into the engine intake, but the technique cannot assess any damage caused by the FOD.
Other methods of detecting impacts on the engine by such as using microphones (as taught in US2011079015) or by vibration and acoustic emissions can help to determine the position and severity of an impact.